1. Field of the Invention
The present invention relates to a mobile node, and more particularly to a mobile node for performing communications on a mobile IP (Internet Protocol) network.
2. Description of the Related Art
With recent advance of the multimedia, a request for an information network is growing sophisticated and variable. In order to meet the request, development of the information network is also growing more rapidly for the purpose of realizing a ubiquitous network society. (The work “ubiquitous” means being or seeming to be everywhere at the same time in the Latin language.) The ubiquitous network is remarked as the next stage of the computerization society. This network is oriented to the ubiquitous computing environment where any computer may be used everywhere at any time.
What is significant to the ubiquitous environment is that a user can connect to the network at any time. Hence, a request has risen for realizing wireless communications with no limitation caused by cable connection. In order to meet the request, today, a mobile node is growing more multifunctional so that a user can connect to a plurality of networks (mobile IP networks) through one mobile node by various kinds of methods.
On the other hand, a key to prevailing the mobile IP is a hand-over technology. The hand-over is a technology of automatically switching a connecting destination of the current network with which a mobile node is communicating to another destination so that if the mobile node is moved to another network, the communication is kept stable.
For example, in a case that a person who listens to a music tune with a PDA (Personal Digital Assistants) provided with a wireless LAN (Local Area Network) is removed from a coffee shop to a boutique located adjacent to the coffee shop (assuming that the PDA and the wireless LAN are matched to the mobile IP), provision of the hand-over function allows the user to listen to the music tune streamingly even if the user moves to another place. This type of hand-over control needs so instant an execution that the user is unconscious of the hand-over. This instant hand-over control is quite important to data communications rather than speech communications.
The description will be oriented to the conventional hand-over technology. In the case of detecting a disconnection of a currently connected network, the conventional hand-over operation is executed to automatically select a predetermined user's policy and a network to be used for the user's policy when switching one network to another. Further, there has also been proposed a technology of measuring an electric field intensity of a received signal, determining the use of a current station in a weak electric field intensity if the measured result is equal to or less than a reference value, and handing the connection over to another station. For example, refer to the Official Gazette of the Japanese Unexamined Patent Publication No. 2000-50337 (paragraph numbers [0005] to [0010] and FIG. 2).
In the foregoing prior arts, however, the former art is arranged to execute the hand-over operation after detecting a disconnection of a currently connected network. It means the communication is interrupted from the disconnection to the completion of switching the current network to another network. Hence, if the switch from one network to another is executed while packets are being received, disadvantageously, a packet loss takes place. For example, while a user enjoys a streaming broadcast, a streaming image or speech is interrupted. If packets of streaming data reach the user's equipment during the interruption, such a packet loss takes place.
On the other hand, the latter prior art (Japanese Unexamined Patent Publication No. 2000-50337) is arranged to execute the hand-over operation not after disconnection of the currently connected network but when the electric field intensity of the received signal is equal to or less than the reference value. No consideration is taken of a process for registering a mobile node when the network is switched. Disadvantageously, therefore, the communication may not be restarted after the hand-over operation.
That is, if the registration process of a mobile IP required when switching the network is not completed on the network side, no communication may be started properly in a new destination network. Hence, the mobile node is required to allow the packets passed through the previous network before being moved to be received for a given length of time even after the hand-over operation.
Further, in some cases, the wireless network may be disconnected once a connection is established (for example, closer to a border between the radio wave receiving areas). For preventing unsmoothness in connecting one network with another, therefore, what is further important is that the hand-over operation is executed after a destination network becomes stable.